Optical time-domain reflectometer capable of determining when a test of an optical fiber should be conducted

ABSTRACT

An optical time-domain reflectometer (OTDR) having a light-emitting element, a branching section, a light-receiving section, and a processing section. The light-emitting element emits pulsating measuring light when driven. The branching section applies the measuring light emitted from the light-emitting element, to an optical fiber to be measured, and branches the light supplied from the optical fiber. The light-receiving section receives the light branched by the branching section and detects a level of the light received. The processing section detects a level of the light supplied from the optical fiber after the OTDR is connected to the optical fiber, while not driving the light-emitting element. The processing section drives the light-emitting element only when the level of the light received is equal to or lower than a preset value.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP00/06025 (not published in English)filed Sep. 6, 2000.

TECHNICAL FIELD

This invention relates to an optical time-domain reflectometer(hereinafter referred to as “OTDR”) that is use search for defectivepoints in an optical fiber. More particularly, the invention relates toan OTDR that does not influence the communication via the line duringmeasuring.

BACKGROUND ART

As is known, an OTDR is connected to one end of an optical fiber andapplies a pulsed light beam into the optical fiber from the end thereof.The OTDR measures the time that lapses until reflected light(backscattered light) arrives from the optical fiber, the level of thereflected light and the like. The losses in the optical fiber, thepositions where the losses occur (e.g., connection points), thepositions of far ends, and the like are thereby determined.

FIG. 6 is a diagram showing how optical fibers are laid and how thevarious characteristics of the optical fibers are measured by aconventional technique.

On a panel 30 a port is provided for a plurality of optical fibers F. Ofthe fibers F, one line is used to achieve communication (i.e., an activeline) is connected to a transmitter 32 a and a receiver 32 b.

For convenience, only one core line of the optical fiber F, or activeline, is illustrated in FIG. 6. In practice, many lines of the opticalfiber F serve as active lines.

An OTDR 31 is connected to one of the optical fibers F that are notused, that is, those other than the active lines, in order to performvarious measurements on that optical fiber F.

The core line that is used as an active line is known and identifiedwith the core-line number and the like. The OTDR 31 is thereforeconnected to a port of a core-line number not assigned to the activeline.

The core-line number may be mistaken for another and the measuring maybe started. In this case, measuring light (pulsed light) is applied intothe core of the active line through which communication is beingachieved. This may influence the communication.

Also, the waveform of the measuring signal changes to an abnormal one.Consequently, normal measuring of characteristics may not beaccomplished.

The panel 30 has hundreds of ports or thousands of ports. Almost allports are in connected state, no matter whether they are used for activelines. To initiate measuring, the ports are disconnected and connectedto the OTDR. Hence, one core-line number may be mistaken for another, asmentioned above.

Once the port of any active line is disconnected, the light being usedis no longer applied, inevitably causing a problem. In this condition,any port that is not of any active line can be identified, only by thecore-line number allocated to the port. This may cause a mistake.

In some cases, a measuring port 33 is provided at a position differentfrom the position where the panel 30 is arranged.

The measuring port 33 is connected to each core line of the fiber F bymeans of a coupler (not shown) or the like. The port 33 is used duringthe measuring only; it is usually opened.

Once the OTDR 31 is connected to the measuring port 33 to effectmeasuring, the core line of the fiber F, which is connected to thetransmitter 32 a and receiver 32 b, cannot be visually detected.

When light is applied to the core line of a wrong number, therebystarting the measuring, the measuring signal will immediately enter thecommunication line, adversely influencing the communication. Inaddition, the measuring side will be affected, resulting in an abnormalwaveform and the like.

DISCLOSURE OF INVENTION

The present invention has been made in order to solve the problemsdescribed above. The object of the invention is to provide an OTDR whichprevents various measuring of an active line effecting communication,which influences neither the communication line nor the measuring andwhich can therefore perform measuring in safety.

According to one aspect of the invention, there is provided an OTDRhaving:

a light-emitting element (2 b) for emitting pulsating measuring lightwhen driven;

branching means (3) for applying the measuring light emitted from thelight-emitting element, to an optical fiber to be measured, andbranching light supplied from the optical fiber; and

light-receiving means (11) for receiving the light branched by thebranching means and detecting a level of the light received.

The OTDR further comprises processing means for detecting a level of thelight supplied from the optical fiber after the OTDR is connected to theoptical fiber, while not driving the light-emitting element, and fordriving the light-emitting element only when the level of the lightreceived is equal to or lower than a preset value.

According to another aspect of this invention, there is provided an OTDRcomprising:

light source driving means;

a light source for emitting pulsating measuring light when driven by thelight source driving means;

branching means for applying the measuring light emitted from the lightsource, to an optical fiber to be measured, and branching light suppliedfrom the optical fiber;

light-receiving means for receiving the light branched by the branchingmeans and detecting a level of the light received;

determining means for causing the light-receiving means to detect thelevel of light received while the light source is not emitting light andfor determining that the optical fiber is not serving as a communicationline, only when the level of light received is equal to or lower than apreset value; and

processing means for causing the light source driving means to drive thelight source, thereby to start the measuring of the optical fiber, whenthe determining means determines that the optical fiber is not servingas a communication line.

According to a further aspect of the invention, there is provided amethod of measuring an optical fiber by use of an optical time-domainreflectometer, the method comprising the steps of:

providing a light source that emits pulsating measuring light whendriven by light source driving means;

providing branching means for applying the measuring light emitted fromthe light source, to an optical fiber to be measured, and branchinglight supplied from the optical fiber;

providing light-receiving means for receiving the light branched by thebranching means and detecting a level of the light received;

causing the light-receiving means to detect the level of light receivedwhile the light source is not emitting light and determining that theoptical fiber is not serving as a communication line, only when thelevel of light received is equal to or lower than a preset value; and

causing the light source driving means to drive the light source,thereby to start the measuring of the optical fiber, when it isdetermined that the optical fiber is not serving as a communicationline.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an OTDR that is the first embodimentof the invention;

FIG. 2 is a flowchart explaining how the OTDR shown in FIG. 1 examinescommunication light;

FIGS. 3A, 3B and 3C are diagrams depicting the measuring waveform of anoptical fiber, detected by the OTDR of FIG. 1;

FIGS. 4A, 4B and 4C are diagrams depicting the measuring waveform of anoptical fiber, detected by the OTDR of FIG. 1;

FIG. 5 is a flowchart explaining the operation of an OTDR that is thesecond embodiment of the invention; and

FIG. 6 is a diagram illustrating how optical fibers are laid, forexplaining the prior art.

BEST MODE FOR CARRYING OUT OF THE INVENTION

The present invention will be summarized first.

To achieve the object described above, an OTDR according to theinvention may be connected to one end of an optical fiber laid in anoptical communication path. The OTDR applies measuring light into theoptical fiber and measures various characteristics of the optical fiberfrom the condition of the light applied back from the optical fiber. TheOTDR is characterized in that it comprises a light source (2), alight-receiving element (4), and processing means (8). The light source(2) emits the measuring light to the optical fiber. The light-receivingelement (4) receives the light reflected from the optical fiber anddetects the level of the light. The processing means (8) inhibits thelight source from emitting the measuring light if the level of light thelight-receiving element has detected before the emission of measuringlight from the light source is higher than a prescribed level.

The OTDR may further comprise a start key (9) and processing means (8).The start key is provided for starting the various characteristics ofthe optical fiber. The processing means (8) detects the level of thelight the light-receiving element (4) detects after the light source (2)has stopped emitting light. If the level is higher than a prescribedlevel, it is determined that the optical fiber is performingcommunication. In this case, the processing means inhibits the emissionof measuring light, thereby to interrupt the measuring ofcharacteristics.

When a power switch is turned on, the processing means (8) may detectthe level of light received at the light-receiving element (4) while thelight source is not emitting light. When the level of light received isequal to or higher than the preset value, the processing means maydetermine that the optical fiber is serving as a communication line andinhibits the emission of the measuring light, thereby interrupting themeasuring of characteristics.

Further, the processing means (8) may generate an alarm informing outputmeans (10) that it is impossible to measure the characteristics of theoptical fiber, when the level of light received, detected at thelight-receiving element while the light source is not emitting light isequal to or higher than the preset value.

In the structure mentioned above, when the start key (9) is depressed tomeasure the characteristics of the optical fiber, the processing means(8) detects the level of light received at the light-receiving element(4), while the mission of light from the light source (2) isinterrupted.

Here, if the level of light received is equal to or higher than thepreset value, the processing means determines that the optical fiber isserving as a communication line and inhibits the emission of themeasuring light, thereby interrupting the measuring of characteristics.At the same time, the processing means generates an alarm informingoutput means 10 that it is impossible to measure the characteristics ofthe optical fiber.

Embodiments of the invention summarized above will be described, withreference to the accompanying drawings.

FIRST EMBODIMENT

FIG. 1 is a block diagram showing an OTDR that is the first embodimentof the invention.

The OTDR 1 is connected to one end of an optical fiber F that has beenlaid in an optical communication path.

A light source 2 has a drive circuit 2 a, a light-emitting element 2 bsuch as a laser diode (LD) or the like. The light-emitting element 2 bemits pulsating measuring light (a laser beam) having center wavelengthλ.

The drive circuit 2 a controls the driving of the light-emitting element2 b.

The pulsating measuring light emitted from the light source 2 is appliedto the optical fiber F through branching means 3.

The branching means 3 is, for example, a directional optical coupler.

The branching means 3 distributes the light reflected from the opticalfiber (Fresnel reflection light or back scattering light) tolight-receiving means 11.

The light-receiving means 11 includes a light-receiving element 4, anamplifier 5, and an A/D converter 6. The light-receiving element 4receives the light reflected from the optical fiber. The amplifier 5amplifies the output of the light-receiving element 4. The A/D converter6 performs analog-to-digital conversion on the output of the amplifier5.

The light-receiving element 4 is composed of a photodiode (PD) or thelike, for receiving the light reflected. The element 4 outputs adetection signal when it receives the light.

The detected signal is amplified by the amplifier 5, subjected to A/Dconversion in the A/D converter 6 and output to processing means 8.

The processing means 8 has a CPU and a memory section including a ROM, aRAM and the like. The processing means 8 controls the componentsprovided in the OTDR 1.

The processing means 8 measures the characteristics of the optical fiberF in a specific sequence to be described later, on the basis of themeasuring program stored in the memory section such as a ROM or thelike.

The characteristics are measured by examining the communication light todetermine whether the optical fiber performs its function.

Operation means 9 comprises keys, which are operated to operate theparameters, operations, display conditions the apparatus requires tomeasure the various characteristics.

Output means 10 receives the data processed in the processing means 8and representing the characteristics of the optical fiber F.

The output means 10 comprises a display, an interface (I/F), and thelike. The display displays characteristics, values set, alarms and thelike. The interface outputs these items of data to external apparatuses.

Under the control of the processing means 8, a timing controller 12controls the drive circuit 2 a, thereby controlling the timing at whichthe light-emitting element 2 b emits the measuring light and the timingat which the A/D converter 6 samples the signal generated when thebackscattered light is detected.

How the processing means 8 of the OTDR 1 examines the communicationlight will be described below.

FIG. 2 is a flowchart explaining how the communication light isexamined.

FIG. 3 and FIG. 4 are diagrams showing the measuring waveform of anoptical fiber, detected by the OTDR 1.

The start key of the operation means 9 is depressed (FIG. 3A), startingthe measuring of ID) characteristics (Step Si). First, the processingmeans 8 measures the level of the light the light-receiving means 11 hasdetected (Step S2). While the means 8 is measuring the level, thelight-emitting element 2 b remains emitting no light (FIG. 3B).

Next, the processing means 8 determines whether the level of the lightreceived has risen above a preset value (Step S3).

The preset value L1 (see FIG. 3C) is higher than the noise level of thesignal detected by the light-receiving element 4 and amplified by theamplifier 5.

A detection signal having a level higher than the preset value may bedetected during a predetermined time (e.g., 1 sec) from the depressingof the start key. In this case (that is, if Yes in Step S3), theprocessing means 8 determines that the optical fiber F is serving as anactive line (or achieving communications) (Step S4).

In this case, the processing means 8 outputs an alarm (alarm display)signal to the display of the output means 10 and the like (Step S5).

Then, the processing means 8 inhibits the light source 2 from emittinglight, thereby interrupting the measuring operation on the optical fiberF (Step S6).

On the other hand, if the detection signal remains at a level lower thanthe preset value (that is, if No in Step S3), the processing means 8determines that the In optical fiber F is not serving as an active line(or achieving communications) (Step S7).

Thus, the measuring of the characteristics of the optical fiber F isstarted when the start key is depressed (Step S8).

In the measuring process, the light-receiving means 11 receives thelight applied from the light source 2 to the optical fiber F and thenreflected from the optical fiber F.

It will be described how the backscattered light from the optical fiberF is measured.

When the measuring light is applied to the optical fiber F at intervalsTR (see FIG. 4C) as shown in FIG. 4B, the light-receiving element 4detects reflected light (backscattered light) that attenuates with thedistance to the optical fiber F as is illustrated in FIG. 4C.

In FIG. 4C, fx indicates the light that has been reflected at the farend of the optical fiber F shown in FIG. 1 and has been detected.

With the above-mentioned process, it is possible to determine thecommunication condition of the optical fiber F before measuring thecharacteristics of the optical fiber F. If the optical fiber F isserving communication, the application of the measuring light to theoptical fiber F is inhibited. This would influence neither thecommunication line nor the measuring.

In the embodiment described above, the start key must be pushed again tomeasure the optical fiber to measure the optical fiber after it isdetermined that the fiber F is serving communication. Nonetheless, thepresent invention is not limited to this embodiment.

For example, the operation may automatically go to Step S2 as shown bythe dotted line in FIG. 2 if the optical fiber F is an active line,thereby effecting continuous detection.

In this case, the measuring of characteristics can be startedimmediately after the communication is interrupted or completed.

The timing of detecting the condition of the communication light is notlimited to the start of measuring the characteristics of the opticalfiber F. Rather, the condition may be detected after the start ofmeasuring the characteristics is started, that is, during the measuringof the characteristics.

In this case, Step S2 and the following steps, shown in FIG. 2, may beautomatically performed during the blank period (TB) shown in FIG. 4C,in which the characteristics are not measured at all.

Measuring light is not applied to the optical fiber F during the blankperiod TB. Thus, it is determined that the fiber F is serving as anactive line, if the light received is at a level equal to or higher thanthe prescribed level. If the light received is at a level lower than theprescribed level, it is determined that the fiber F is not serving as anactive line. In either case, an appropriate process is performed.

If it is determined that the fiber F is an active line, Step S6 and thefollowing steps shown in FIG. 2 are performed to interrupt themeasuring, as is indicated by the solid line in FIG. 2. Alternatively,the process returns to Step S2 as indicated by the dotted line, therebyto detect again the level of the light received.

SECOND EMBODIMENT

The second embodiment of the invention will be described.

An OTDR 1, which is the second embodiment of the invention, will bedescribed.

The OTDR 1 according to the second embodiment is similar in structure tothe first embodiment shown in FIG. 1.

FIG. 5 is a flowchart explaining the operation of the second embodiment.

The steps of the operation will be designated by identical symbols andwill not be described.

First, the power switch to the OTDR 1 is turned on (Step S10). Theprocessing means 8 enters into a standby state (Step S11).

In the standby state, various initial values are set which are requiredfor measuring the characteristics of the optical fiber F.

In the standby state, the level of the light received at thelight-receiving element 4 is measured, while interrupting the emissionof light from the light source 2.

Thereafter, it is determined whether the optical fiber is an active lineor not (Steps S3 to S8).

After the power switch is turned on, it may be determined that the fiberF is not an active line (Step S7). In this case, it is determinedwhether the start key has been depressed to measure the characteristicsof the optical fiber (Step S12). When the start key is depressed (if YESin Step S12), the measuring of the characteristics of the optical fiberis immediately started.

After the power switch is turned on, it may be determined that the fiberF is an active line (Step S4). If this is the case, the application ofmeasuring,light is inhibited (Step S6) and the process returns to StepS11.

Thus, the standby state continues as long as the fiber F is regarded asan active line. When the start key is depressed (if YES in Step S12)after the fiber F is found to be no longer an active line, the measuringof characteristics of the optical fiber can be started at once.

The OTDR 1 according to this embodiment automatically determines whetheran active line is available or not, when the power switch is turned on,and waits for the start of characteristic measuring.

The present invention is not limited to the embodiment described above.Rather, various modifications can be made.

An optical path control means 20 such as a shutter may be provided asindicated by dotted lines in FIG. 1, for opening and closing the opticalpath between the light-emitting element 2 b and the branching means 3 inaccordance with the output of the amplifier 5. In this case, it ispossible to prevent the application of measuring light to the activeline, even if the light-emitting element 2 b remains driven at alltimes.

That is, to measure the various characteristics of the optical fiber,the optical path control means 20 first closes the optical path betweenthe light-emitting element 2 b and the branching means 3.

While the output of the amplifier 5 remains at a value which correspondsto the level of the light received and which is equal to or higher thana preset level, the means 20 inhibits the application of measuring lightfrom the optical path.

When the output of the amplifier 5 remains at a value that correspondsto the level of the light received and is equal to or lower than thepreset level, the optical path control means 20 opens the optical pathbetween the light-emitting element 2 b and the branching means 3. Thisallows the application of measuring light from the optical path.

As described above in detail, the application of measuring light isinhibited in the structure of this invention in the process of measuringthe various characteristics of an optical fiber, if the light-receivingelement receives light at a level equal to or higher than a presetlevel. The application of measuring light to an active line servingcommunication can therefore be prevented.

The characteristics can therefore be smoothly measured in safety,without influencing the communication and the measuring.

The communication light may be examined when the start key is operatedto start the measuring of the various characteristics of the opticalfiber. In this case, it is possible to detect the communicationcondition on the line when the measuring is started. Thus, the measuringof characteristics can be carried out in safety.

Further, the communication light may be examined when the power switchis turned on. If this is the case, the communication condition of theline is detected every time the switch is turned on, thereby toaccomplish the measuring of characteristics in safety.

An alarm may be output while the line is serving the communication,informing the operator that it is now impossible to measure thecharacteristics of the line.

What is claimed is:
 1. An optical time-domain reflectometer (OTDR)comprising: a light-emitting element for emitting pulsating measuringlight when driven, branching means for applying the measuring light fromthe light-emitting element to an optical fiber to be measured, and forbranching light supplied from the optical fiber, light-receiving meansfor receiving the light branched out by the branching means anddetecting a level of the light received, detecting means for detecting alevel of the light supplied from the optical fiber after the OTDR isconnected to the optical fiber, while not-driving the light-emittingelement, and processing means for driving the light-emitting elementonly when the level of the light detected by the detecting means isequal to or lower than a preset value, when the optical fiber is notserving as a communication line.
 2. The OTDR according to claim 1, whichfurther comprises a start key for starting measurement of variouscharacteristics of the optical fiber, and wherein when the start key isoperated the processing means detects the level of light received at thelight-receiving means while the light-emitting element is not emittinglight, and when the level of light received is equal to or higher thanthe preset value, the processing means determines that the optical fiberis serving as a communication line and inhibits the emission of themeasuring light.
 3. The OTDR according to claim 1, wherein when a powerswitch is turned on, the processing means detects the level of lightreceived at the light-receiving means while the light-emitting elementis not emitting light, and when the level of light received is equal toor higher than the preset value, the processing means determines thatthe optical fiber is serving as a communication line and inhibits theemission of the measuring light.
 4. The OTDR according to claim 1,wherein the processing means generates an alarm indicating that it isimpossible to measure characteristics of the optical fiber, when thelevel of light received, detected at the light-receiving means while thelight-emitting element is not emitting light, is equal to or higher thanthe preset value.
 5. The OTDR according to claim 1, which furthercomprises a start key for starting measurement of variouscharacteristics of the optical fiber, and wherein when the start key isoperated, the processing means detects the level of light received atthe light-receiving means while the light source is not emitting light,and when the level of light received is equal to or higher than thepreset value, the processing means determines that the optical fiber isserving as a communication line and inhibits the emission of themeasuring light.
 6. An optical time-domain reflectometer (OTDR)comprising: light source driving means; a light source for emittingpulsating measuring light when driven by the light source driving means;branching means for applying the measuring light emitted from the lightsource to an optical fiber to be measured, and for branching lightsupplied from the optical fiber; light-receiving means for receiving thelight branched by the branching means and detecting a level of the lightreceived; determining means for causing the light-receiving means todetect the level of light received while the light source is notemitting light and for determining that the optical fiber is not servingas a communication line, only when the level of light received is equalto or lower than a preset value; and processing means for causing thelight source driving means to drive the light source, thereby to startmeasuring characteristics of the optical fiber, when the determiningmeans determines that the optical fiber is not serving as acommunication line.
 7. The OTDR according to claim 6, wherein when apower switch is turned on, the processing means detects the level oflight received at the light-receiving means while the light source isnot emitting light, and when the level of light received is equal to orhigher than the preset value, the processing means determines that theoptical fiber is serving as a communication line and inhibits theemission of the measuring light.
 8. The OTDR according to claim 6,wherein the processing means generates an alarm indicating that it isimpossible to measure characteristics of the optical fiber, when thelevel of light received, detected at the light-receiving means while thelight source is not emitting light, is equal to or higher than thepreset value.
 9. A method of measuring characteristics of an opticalfiber using an optical time-domain reflectometer, said methodcomprising: providing a light source that emits pulsating measuringlight when driven by light source driving means; providing branchingmeans for applying the measuring light emitted from the light source toan optical fiber to be measured, and for branching light supplied fromthe optical fiber; providing light-receiving means for receiving thelight branched by the branching means, and for detecting a level of thelight received; causing the light-receiving means to detect the level oflight received while the light source is not emitting light anddetermining that the optical fiber is not serving as a communicationline, only when the level of light received is equal to or lower than apreset value; and causing the light source driving means to drive thelight source, thereby to start the measuring of the characteristics ofthe optical fiber, when it is determined that the optical fiber is notserving as a communication line.
 10. An optical time-domainreflectometer (OTDR) adapted to be provided at one end of an opticalfiber laid in an optical communication line, and to apply measuringlight to the optical fiber and measure various characteristics of theoptical fiber based on light -reflected from the optical fiber, saidOTDR comprising: a light source for emitting the measuring light to theoptical fiber; a light-receiving element for receiving the lightreflected from the optical fiber; detecting means for detecting a levelof the light reflected in accordance with the light received by thelight-receiving element; and processing means for inhibiting emission ofthe measuring light when the level of light detected by the detectingmeans before the measuring light is emitted from the light source isequal to or higher than a preset value, when the optical fiber isserving as a communication line.
 11. The OTDR according to claim 10,which further comprises a start key for starting measurement of variouscharacteristics of the optical fiber, and wherein when the start key isoperated, the processing means detects the level of light received atthe light-receiving element while the light source is not emittinglight, and when the level of light received is equal to or higher thanthe preset value, the processing means determines that the optical fiberis serving as a communication line and inhibits the emission of themeasuring light.
 12. The OTDR according to claim 10, wherein when apower switch is turned on, the processing means detects the level oflight received at the light-receiving element while the light source isnot emitting light, and when the level of light received is equal to orhigher than the preset value, the processing means determines that theoptical fiber is serving as a communication line and inhibits theemission of the measuring light.
 13. The OTDR according to claim 10,wherein the processing means generates an alarm indicating that it isimpossible to measure characteristics of the optical fiber, when thelevel of light received, detected at the light-receiving element whilethe light source is not emitting light, is equal to or higher than thepreset value.
 14. An optical time-domain reflectometer (OTDR)comprising: a light-emitting element for emitting pulsating measuringlight when driven; branching means for applying the measuring light fromthe light-emitting element to an optical fiber to be measured, and forbranching light supplied from the optical fiber; light-receiving meansfor receiving the light branched out by the branching means anddetecting a level of the light received; processing means for detectinga level of the light supplied from the optical fiber after the OTDR isconnected to the optical fiber, while not driving the light-emittingelement, and for driving the light-emitting element only when the levelof the light received is equal to or lower than a preset value; and astart key for starting measurement of various characteristics of theoptical fiber; wherein when the start key is operated, the processingmeans detects the level of light received at the light-receiving meanswhile the light source is not emitting light, and when the level oflight received is equal to or higher than the preset value, theprocessing means determines that the optical fiber is serving as acommunication line and inhibits the emission of the measuring light. 15.An optical time-domain reflectometer (OTDR) comprising: a light-emittingelement for emitting pulsating measuring light when driven; branchingmeans for applying the measuring light from the light-emitting elementto an optical fiber to be measured, and for branching light suppliedfrom the optical fiber; light-receiving means for receiving the lightbranched out by the branching means and detecting a level of the lightreceived; and processing means for detecting a level of the lightsupplied from the optical fiber after the OTDR is connected to theoptical fiber, while not driving the light-emitting element, and fordriving the light-emitting element only when the level of the is lightreceived is equal to or lower than a preset value; wherein when a powerswitch is turned on, the processing means detects the level of lightreceived at the light-receiving means while the light source is notemitting light, and when the level of light received is equal to orhigher than the preset value, the processing means determines that theoptical fiber is serving as a communication line and inhibits theemission of the measuring light.
 16. An optical time-domainreflectometer (OTDR) adapted to be provided at one end of an opticalfiber laid in an optical communication line, and to apply measuringlight to the optical fiber and measure various characteristics of theoptical fiber based on light reflected from the optical fiber, said OTDRcomprising: a light source for emitting the measuring light to theoptical fiber; a light-receiving element for receiving the lightreflected from the optical fiber and detecting a level of the lightreflected; processing means for inhibiting emission of the measuringlight when the level of light detected by the light-receiving elementbefore the measuring light is emitted from the light source is equal toor higher than a preset value; and a start key for starting measurementof various characteristics of the optical fiber; wherein when the startkey is operated, the processing means detects the level of lightreceived at the light-receiving element while the light source is notemitting light, and when the level of light received is equal to orhigher than the preset value, the processing means determines that theoptical fiber is serving as a communication line and inhibits theemission of the measuring light.
 17. An optical time-domainreflectometer (OTDR) adapted to be provided at one end of an opticalfiber laid in an optical communication line, and to apply measuringlight to the optical fiber and measure various characteristics of theoptical fiber based on light reflected from the optical fiber, said OTDRcomprising: a light source for emitting the measuring light to theoptical fiber; a light-receiving element for receiving the lightreflected from the optical fiber and detecting a level of the lightreflected; and processing means for inhibiting emission of the measuringlight when the level of light detected by the light-receiving elementbefore the measuring light is emitted from the light source is equal toor higher than a preset value; wherein when a power switch is turned on,the processing means detects the level of light received at thelight-receiving element while the light source is not emitting light,and when the level of light received is equal to or higher than thepreset value, the processing means determines that the optical fiber isserving as a communication line and inhibits the emission of themeasuring light.